This is a proposal to define the immunoregulatory capabilities of anergic B cells. Anergy in B cells is a state of nonresponsiveness resulting from chronic stimulation through the B cell receptor (BCR), occurring in the absence of T cell help and/or innate stimulation. It is a major form of tolerance among self-reactive B cells. Anergic B cells are widely viewed as nothing more than slowly dying casualties of immunological self-tolerance, and as liabilities to the physiological immune system under circumstances where anergy might be reversed by costimulation or inflammation. To the contrary, we recently found that anergic B cells are potent regulators of immunity. Our initial observation was made upon a routine adoptive transfer involving anergic Ars/A1 B cells. Ars/A1 B cells express a dual-reactive transgenic BCR that binds the hapten p- azophenylarsonate (Ars) and single-stranded DNA, which stably maintains their anergic state upon adoptive transfer. We found that when wildtype adoptive recipients of Ars/A1 B cells were immunized with foreign proteins conjugated with Ars, not only did the Ars/A1 cells fail to mount an antibody response, they also profoundly inhibited the anti-Ars antibody response by the host immune system. This inhibition was systemic, potent (10-30 fold), antigen-specific, applied to the protein carrier, applied to the secondary immune response and required relatively few Ars/A1 B cells. On the basis of this discovery, we hypothesize that anergic B cells play a natural role in maintaining self-tolerance among CD4+ T cells. In this application, we propose to use the Ars/A1 model to define the regulatory capabilities of anergic B cells. The results of this project will almost certainly open new avenues of investigation that could lead to a paradigm shift, and to novel strategies to control autoimmunity and possibly transplant rejection. The immune system is an indispensable component of physiology that protects the body from infections, which would otherwise kill us in a matter of hours. A large number of human diseases, however, are caused by cases of mistaken identity, in which lymphocytes of the immune system confuse self-tissues with foreign pathogens. These "autoimmune diseases" include multiple sclerosis, type I diabetes, arthritis, systemic lupus erythematosus, myasthenia gravis, graves disease, Hashimoto's thyroiditis, pemphigus vulgaris, celiac disease, ulcerative colitis, scleroderma, autoimmune uveitis, hemolytic anemia and many others that spare no organ system. Often these autoimmune diseases include pathological antibodies termed autoantibodies, which bind to, and destroy cells and tissues of the body. Antibodies and their pathological counterparts are produced by B lymphocytes. Normally the immune system incapacitates those B lymphocytes that might otherwise produce autoantibodies in a process termed "self-tolerance". Approximately half of the time this means death to the autoreactive B cell. However, in the other half, it means that the cell is allowed to persist in an inactivated or "anergic" state. Scientists who study the immune system have been continually puzzled by anergy, as it seems to present a liability: what if the self-reactive B cell is awakened from its anergic state? Recently we have obtained rather surprising evidence that anergic B cells may play an important role in preventing autoimmunity. We think that they accomplish this by deceiving and inactivating other critical cells that would otherwise participate in the autoimmune reaction against self-tissue, and we want test this idea in a very well-defined genetically-engineered animal model, using an array of specialized molecular tools. We believe our study has the potential to result in a paradigm shift away from the idea that anergic B cells are only dangerous and towards the idea that they are protective. If our idea is correct, it will be possible to use the information gained from this study to design strategies that exploit anergic B cells to preclude or ameliorate autoimmune disease. Moreover, because lymphocytes also reject transplanted tissue, our discoveries may open a clinical avenue to prevent immune rejection of transplanted tissues and organs.